The observable properties of active galactic nuclei (AGN) and black hole X-ray binaries (BHXRBs) are consequences of accretion on to a black hole at a vari- ety of rates, in a variety of ‘states’, and within a variety of environments. The major difference between the aforementioned classes of object is the black hole mass. BHXRBs typically have a black hole mass ∼10M⊙ while for AGN it is 105M⊙ ≤ M ≤ 1010M⊙. Theoretically, the central accretion processes should be relatively straightforward to scale with mass, and this is supported by several ob- served correlations. These include a relation between the X-ray and radio luminosi- ties and the black hole mass (Merloni, Heinz & di Matteo 2003; Falcke, K¨ording & Markoff 2004), and between X-ray variability timescales, mass accretion rate and mass (McHardy et al. 2006). More qualitative similarities between accretion ‘states’ and radio jet production have also been demonstrated (K¨ording, Jester & Fender 2006; for the current picture of accretion states in BHXRBs and their rela- tion to radio jets see Fender, Belloni & Gallo 2004).
Studying the delays between different emission regions gives us a further handle on the scalability of black hole accretion, as signals propagate from, for example, the accretion flow to the jet. Variability studies have so far shown that a correlation exists between the X-ray and optical emitting regions of both BHXRBs and AGN, typically reporting small lags, which are consistent with at least some of the optical variations being due to X-ray heating of the disc (Russell et al., 2009; Breedt et al., 2009). A recent study by Casella et al. (2010) has shown that a correlated time lag of∼ 100 ms exists between the X-ray and IR regions (IR lagging X-rays) for the BHXRB GX339-4, indicating a close coupling between the hot accretion flow and inner regions of the jet. In the case of the BHXRB GRS 1915+105 a variable X-ray to radio lag of∼ 30 mins (radio lagging X-ray) has been measured (Pooley & Fender, 1997; Fender et al., 1999). Discrete ejection events have also been resolved in both the AGN and BHXRBs, for examples see Marscher et al. (2002) and Pooley & Fender (1997); Fender et al. (1999).
The linear scaling with mass of the characteristic timescale around a black hole means that there are advantages to studying each class of object. In BHXRBs we can track complete outburst cycles, from the onset of disc instabilities through major ejection events, radio-quiet disc-dominated states, and a return to quiescence, on humanly-observable timescales (typically years). For a typical AGN the equivalent cycle may take many millions of years. However, for an AGN we are able to resolve individual variations of the source on time-scales that are comparable to or shorter
than the shortest physical time-scales in the system (e.g. the dynamical time-scale), something which is currently impossible for BHXRBs. In ‘black hole time’ we are able to observe the evolution of sources in fast-forward for BHXRBs and in detailed slow-motion for AGN.
In this chapter I present the results of a long term (∼ 3 years) regular monitoring campaign in the X-ray and radio bands of the low luminosity active galactic nucleus (LLAGN) NGC 7213. Previous X-ray studies show that NGC 7213 is accreting at a low rate ∼ 7 × 10−4 LEdd (Starling et al., 2005). The hard state in BHXRBs is typically observed at bolometric luminosities below∼ 1% Eddington, and seems to be ubiquitously associated with a quasi-steady jet. Above∼ 1%, sources can switch to a softer X-ray state, the jets are suppressed (Maccarone, Gallo, & Fender, 2003; Dunn et al., 2009); furthermore transition to this softer state is usually associated with major transient ejection events. As NGC 7213 is considerably below LEdd ∼ 1% I therefore consider it a good candidate for comparison with other BHXRBs in the low/hard state. If I consider AGN to be ‘scaled up’ versions of BHXRBs by exploring the time lag between the X-ray and radio emitting regions I can compare, contrast and hopefully relate the accretion and jet production scenarios for AGN and BHXRBs.
A correlation has been established by Corbel et al. (2003) and Gallo et al. (2003, 2006) relating the radio luminosity (LR) and X-ray luminosity (LX) for BHXRBs in the low/hard and quiescent states, where LR ∝L0X.6−0.7. Merloni, Heinz, & di Mat- teo (2003) - hereafter MHdM03 and Falcke, K¨ording, & Markoff (2004) extended the BHXRB relationship using two samples of AGN to form the ‘fundamental plane of black hole activity’. By accounting for the black hole mass (M) the relationship LR ∝L0X.6−07 has been extended to cover many orders of magnitude in black hole mass and luminosity i.e LR = (0.6+0.11−0.11) log LX + (0.78+0.11−0.09) log M + 7.33+4.05−4.07. Further refinements were made to the fundamental plane by K¨ording, Falcke, & Corbel (2006) - hereafter KFC06, using an augmented and updated sample to ex- amine the fitting parameters (also see G¨ultekin et al. 2009 and Broderick & Fender 2011).
Throughout this chapter I define the ‘intrinsic’ behaviour of AGN and BHXRBs as multiple measurements (in the radio and X-ray) of the same source. I define the ‘global’ behaviour as single (or average) measurements of multiple sources, both with respect to the fundamental plane.
For the BHXRBs in the low/hard state the relationship described above has not only been established globally but in some cases intrinsically, i.e. GX 339-4, V404 Cyg and a small number of other systems have been shown to move up and down the
correlation seen in the fundamental plane (Fender et al., 2007). However, in recent years an increasing number of outliers have been found below the correlation, i.e. less radio-loud then expected (Xue & Cui 2007; Gallo 2007; Soleri et al. 2010; Calvelo et al. 2010; Rushton et al. 2010) as well as some sources which move in the plane with a different slope (e.g. Jonker et al. 2010 and Coriat et al. 2011). To date the correlation found from the fundamental plane has only been measured globally for AGN, not intrinsically. Note, with respect to the global measurements of the AGN population, the specific measurements of the radio and X-ray flux used in the correlation are sometimes taken at different times and thus could be a source of error in the correlation (K¨ording, Falcke, & Corbel, 2006).
As well as establishing the time lags, another goal of this work, is to establish, through quasi-simultaneous observations the intrinsic relationship between LR, LX and M observed in a LLAGN and its relevance to the fundamental plane of black hole activity. I use the MHdM03 and KFC06 samples for comparison both with an updated BHXRB sample taken from Fender, Gallo & Russell (2010) - hereafter FGR10. I also explore the possible scatter in AGN data points away from the fun- damental plane and place limits on this deviation.